This invention relates to controlling the parameters of a variable speed motor drive in response to the temperature of the solid state switches within the drive.
Solid state devices used in motor drives, typically insulated gate bipolar transistors (IGBTs) and their anti-parallel diodes, undergo thermal stress as a function of the load on the motor and the switching frequency in the drive. Particularly, thermal differences between the solid state junctions and the thermal sink create mechanical stress and ultimate failure of bonded wires. Exemplary motor drives are disclosed in U.S. patent applications Ser. No. 09/310,393 and Ser. No. 09/310,31 1. Since thermal stress of solid state devices results in failure thereof, drives are typically designed to carry more than the anticipated load so as to reduce the possibility and delay the onset of failure, by reducing thermal stresses which occur during the worst-case drive situations.
The zero state of space vector, zero state, pulse width modulation motor drives is when there is no differential voltage driving the motor (although there may be current flow); then, all upper switches of the matrix are conducting, and all lower switches are off, so all lower diodes are conducting, or vice versa. It is known that transistor switching losses in motor drives are much higher than diode switching losses or conduction losses of either diodes or transistors. Motor drives are conventionally controlled so that the zero state is selected to minimize transistor switching, thereby to reduce losses and thermal stress in the transistors. However, such operation leaves selected diodes in a conducting state for long time segments, which must be accommodated in the design of the motor drive. Although switching losses and therefore device temperatures can be reduced by using lower switching frequencies, such lower switching frequencies produce noise which is objectionable in certain environments, such as elevators.
Objects of the present invention include avoiding excessive thermal stress in solid state switches utilized in variable speed motor drives; avoiding excessive junction/heatsink (chassis) temperature differentials in such drives; predicting lifetime and failure modes of solid state switches in variable speed motor drives; utilizing variable speed motor drives maximally without creating excessive thermal stresses therein; reducing mechanical failure of bonded wires in such drives; improving variable motor drive operation; and providing variable speed motor drives that can automatically adjust operation to suit various loads and to suit different utilizations.
According to the present invention, the approximate temperatures of solid state switching devices in variable speed motor drives, such as elevator motor drives, including insulated gate bipolar transistors (IGBTs) and diodes, are determined, either by measurement or by estimation; transistor and diode conduction and switching losses, thermal resistance and transient thermal response between each device and the case of each device, and between each case and a thermal sink, as well as normalized case-to-sink transient thermal response, are used to form models of heat flow and temperature as a function of current flowing through the devices, duty cycle of the devices, and device switching frequency. When transistor temperatures exceed a threshold, device operation is altered, such as by lowering the switching frequency or reducing the maximum load, such as by reducing acceleration; when transistor temperatures fall below a threshold, device operation is altered, such as by raising the switching frequency or the maximum load. According further to the invention, the choice between a preferred zero state which does not incur transistor switching losses, and an alternative zero state which will incur transistor switching losses, is made on a time share basis, with the duty cycle of choice for the non-preferred zero state being based upon diode junction temperature. When the highest diode temperature exceeds a threshold, the percent of time in which the zero state involving that diode is selected to be the alternative zero state, in which that diode will not conduct, is increased so as to permit the diode temperature to be lowered; but when that diode temperature falls below the threshold, then the percent of time in which the preferred zero state is selected is increased.
According still further to the invention, the slew rate (rate of change with time) of decreasing temperature, and therefore of increasing switching frequency, maximum load or diode duty cycle, is limited to prevent changes from following motor speed and/or load variations; the slew rate limit may be a function of motor speed; and upper and lower limits may be applied to switching frequency or maximum load and to diode duty cycle. Temperature information may be accumulated over time to predict device life, thereby to permit replacement only as realistically needed. The motor drive can thereby custom design its operating characteristics to suit any utilization to which it is put, e.g., one type of elevator or another type of elevator, as well as to alter its operation on a use-by-use basis so as to suit the duty imposed during each use of the motor, such as each run of an elevator.
In accordance still further with the invention, the alterations to operation described hereinbefore may be proportional to temperature excesses.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.